As semiconductor integrated circuit devices and uses therefor have become increasingly complicated, it has become more difficult to use a single metal level to make all of the electrical connections to and between the devices in an integrated circuit. The need for multiple levels of interconnecting metallizations has made integrated circuit fabrication more difficult, for several reasons.
For example, it is generally desirable to begin any patterning step with a relatively smooth surface, because a smooth surface facilitates accurate pattern transfer and also facilitates forming continuous portions of metallization patterns thereover. Unfortunately, structures with two or more levels of metals typically have surfaces with complex topography, i.e., nonplanar surfaces. To ameliorate the adverse effects of such nonplanarity, integrated circuit processing sequences typically include a step which provides a smooth surface of, for example, a dielectric. A typical such sequence involves depositing a dielectric layer, such as a glass which is relatively conformal to the nonplanar topography, and then either flowing it or etching it back to form a relatively smooth surface.
Although the aforementioned sequence produces a relatively planar surface, other problems often arise. For example, such a sequence can introduce many possible contaminants which can adversely affect circuit performance, and especially troublesome adverse effects are caused by the presence of contaminants which produce mobile ions, typically sodium ions, in the dielectric layer. Unfortunately, many potential sources of mobile ions are present during semiconductor integrated circuit fabrication, and they are not easily eliminated. As will be readily appreciated by those skilled in the art, such mobile ion contamination often can be at levels which are high enough to result in an undesirably large number of integrated circuit failures or unsatisfactory performance.
Several approaches have been taken heretofore to reduce the effects of contamination. One such approach uses a gettering agent in the contaminated dielectric layer. The gettering agent is effective, after heating the dielectric layer to a sufficiently elevated temperature, typically greater than about 800.degree. C., to reduce the effects of contaminants by trapping and holding them in the form of stable, charge-neutralized compounds. This technique is often used with a phosphorus-containing dielectric layer deposited prior to the first level metal. Such a dielectric typically flows at temperatures within a range between about 800.degree. C. to 1000.degree. C. At these temperatures, the phosphorus present in the dielectric layer acts as a gettering agent for mobile ions which are present therein.
In contemporary integrated circuit fabrication, several levels of metal may be deposited, and each deposition step typically is followed by pattern delineation. However, the material properties of the first level metal typically dictate a temperature, e.g., about 450.degree. C. for aluminum, which cannot be exceeded during the remainder of the process sequence without destroying the integrity of the first level metal. Unfortunately, this maximum temperature is much lower than the 800.degree. C. to 1000.degree. C. temperature which is effective in the initial flowing and gettering process, i.e., prior to the first level metal.
It will be readily appreciated that many of the sources of mobile ion contamination necessarily occur at processing steps subsequent to forming the first level metal. Thus, due to the limitation on the maximum temperature imposed by the material properties of the first level metal, the above-described gettering step, which was used to neutralize contamination from the first level dielectric before forming the first level metal, typically cannot be used to remove mobile ions from subsequently deposited dielectric layers.